Compare this with JOSM, where if you zoom in although there is a "no tiles at this zoom level" message, the previous level's tiles are magnified:

A quick search of trac doesn't find a matching enhancement request for it, but that's probably where it's best logged (as an enhancement against component Potlatch2, with a relatively low priority, as nothing's actually broken).

Exactly right. I would say, though, that I'm not aware of any intention by developers to address this in P2 (though I've been proved wrong before!), and certainly as the maintainer it's not something I'm planning to work on. Other imagery providers, such as MapQuest Open Aerial and indeed much OSM-hosted imagery, offer serverside scaling. As Vincent de P says, in many areas Bing are commissioning new high-res imagery anyway.

JOSM appears to do automatic scaling (presumably client-side) - I suspect it tries to get imagery at zoom 17, fails, but then says "aha! I'll use the imagery that I have at zoom 16 and just make it bigger". It's not a "limit" as such, just a feature that no-one's added yet (and, from Richard's comment, no-one's planning to).

Bing Zoom 22 imagery exists for high-tech-central in England but not for rural France because they're following the money - see vincent-de-phily's links in the other answer.